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This is exactly the feeling I had when I played Assassin's Creed and picked up a flag that said "1/100". That happened multiple times, since there are 400 flags in the game. And what do you get? Absolutely nothing but an achievement.

I hold Mario Odyssey up as a shining example of how to make large optional objectives fun. You don't really get much of a reward for getting all 999 moons, but at least the vast majority of them have fun puzzles to solve so that it's actually rewarding to collect them. Contrast this with, say, Korok seeds.